Tech Directions - May 2008 - (Page 18)

Student measures and glues his balsa wood structure while a classmate (below) checks her tower side’s total weight on a triple beam balance. times their own weight. To drive home the point that their little structures are very strong, I tell students that even though the world’s strongest weightlifters can clean and jerk 579.8 lb. (263 kg), their strength-toweight ratio is still only about 2.5:1 —pretty puny by structural standards. Student Procedure Students first construct a practice balsa square (Fig. 1). When they can demonstrate that they understand how to join materials, they can move on to build a balsa side using 23 structural members on the pattern I supply (Fig. 3). Again, while they don’t have much room for creativity here, using a uniform design allows them to make comparisons between each other’s craftsmanship and determine who the best “structural engineer” is. Once students have tested and analyzed their failed side, I provide them with a “bare bones” final design (Fig. 4) to which they add members for the final test, which will give each student the opportunity to apply concepts learned earlier. “clamps” to hold the members in place while the white Elmer’s-type glue dries. Thus, rather than have students build full four-sided balsa towers, they build and test a single side. This saves on both time and materials. To test their structures, students use my Big Buster tester, which I designed specifically for testing two-dimensional balsa structures. In using Big Buster, students observe the effects of a load placed on a lightweight structure. Their goal in building the tower side is to produce a structure that will fail under a heavy load with the highest strength-to-weight ratio. The term that I use for this ratio is structural efficiency. It refers to holding up the maximum load with a minimum of materials. Most of the tower sides that my students build weigh between 2.5 and 4 grams. Students are quick to learn that not all balsa sticks weigh the same and that some of the material is quite weak under compression yet relatively strong under tension. I encourage students to test the rigidity of the balsa that they select with a Pitsco Timber Tester. I limit students to three pieces of 3'-long material. They use the most rigid pieces for columns and the least rigid for ties or diagonal braces. Even though the structures may weigh only a few grams, some have supported loads well in excess of 18,000 grams. Even poorly made structures can support 500–1,000 Performing the Test Before testing their structures, students number each joint so that they can identify which joints or members fail. They then place their structure in Big Buster and lower Fig. 3— Balsa side pattern Fig. 4— Basis of final tower design Maximum of 30 structural members Tower design must be symmetrical 18 techdirections ◆ MAY 2008

Table of Contents Feed for the Digital Edition of Tech Directions - May 2008

Tech Directions - May 2008 Contents Technically Speaking Direct from Washington The News Report Mastering Computers Technology's Past Technology Today Road to Success - Service Learning Enhances Tech Ed Experience Balsa Tower Walls Brave 'Big Buster' These Kids Are Really on the Ball - Inventors Competition Winners School Web Site of the Month Animator Career Exploration Tools Guide Index to Volume 67 More than Fun

Tech Directions - May 2008

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